# [Pw_forum] How to create Vacuum

Mon Jul 24 22:51:19 CEST 2017

```Dear Eduardo,

ESM is one of a variant of a slab calculation. So, you need to put an appropriate thickness of vacuum for a slab calculation. As you know, the criterion of the minimum thickness of vacuum is sufficiently thick enough to avoid the overlap between the wave function (or charge density) from both sides of the slab.

If the thickness is 16AA in your model, then you can use this value as the total thickness of the vacuum region in ESM calculation. So you need to put 8AA thickness vacuum on both sides of the slab. The length of the unit cell along z-direction is (8 + slab thickness + 8) AA.

More precisely, we need to put a sufficiently thick vacuum on both sides to ensure that the wave function (or charge density) decays to zero at the cell edge (z=L/2).

Best regards,
Minoru

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Minoru Otani
National Institute of Advanced Industrial Science and Technology
Research Centre for Computational Design of Advanced Functional Materials
email : minoru.otani at aist.go.jp
tel : +81-29-861-5202
fax : +81-29-861-3171
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On Jul 24, 2017, at 23:36, Eduardo Menendez <eariel99 at gmail.com> wrote:

I think the answer to how to create vacuum is in the example ESM_example.

Let me present a related question. How much can we reduce the width of vacuum in the unit cell when using esm_bc='bc1'?

Eduardo Menendez Proupin
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